using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {       

        var tests = new List<(int, int[])>() {

            (5, new [] {1, 3, 6, 4, 1, 2}),

            (4, new [] {1, 2, 3}),

            (1, new [] {-1, -3}),

            (3, new [] {1, -3, 6, 4, 5, 7, -1, 2}),

            (1, new int[] {})

        };

        foreach (var test in tests) {

            Console.WriteLine(string.Format("[{0}] = {1}, expected {2}.", string.Join(",", test.Item2), solution(test.Item2), test.Item1));

        }

    }

    public static int solution(int[] A)

    {

        var B = A.Distinct().Where(a => a > 0).ToArray();

        Array.Sort(B);

        var n = B.Length;

​

        for (var i = 0; i < n; i++)

        {

            if (B[i] != i + 1)

            {

                return B[i] - 1;

            }

        }

        return n + 1;

    }

}